Method and apparatus for sensing the force with which a button is pressed

ABSTRACT

Disclosed is an apparatus for sensing a force, comprising an actuator having a conductive deformable surface, a substrate having a first conductive trace and a second conductive trace, a housing coupled to the actuator and to the substrate, holding the actuator in proximity to the substrate, and a circuit for measuring a capacitance value.

TECHNICAL FIELD

The present invention relates generally to electronic circuits and in particular to circuits for sensing force.

BACKGROUND

Force-sensing buttons have found recent widespread use in human interface devices such as gamepads for the entertainment consoles like the Sony PlayStation™ and Microsoft Xbox™. A conventional gamepad 100 is shown in FIG. 1. The conventional gamepad 100 comprises a housing 110, having four force-sensing triggers 120, a D-pad 130 with four force sensing buttons controlled by a left hand, four force sensing buttons 140 controlled by a right hand, and two thumbsticks 150 controlled by thumbs. The force sensing buttons comprise electronic force sensing actuators (in which the force applied to a button is sensed, rather than the binary state of a button) to provide variable force inputs to the console. In this conventional gamepad, there are twelve force sensing buttons/actuators. Typically each force sensing button/actuator output is translated in a six or eight bit value representing the force applied.

One conventional implementation for a force sensing actuator is the use of a force sensing resistor, such as those sold by Interlink Electronics (cited in information disclosure statement). However the force sensing resistor solution is too expensive for many applications where cost is an important factor. Many purchasers of gamepads and other consumer products are very price sensitive, so having a low manufacturing cost is important.

Another lower cost conventional implementation (which has been adopted by many gamepad manufacturers) is to use a resistive track printed on a printed circuit board (PCB). Printed circuit boards typically comprise a substrate, with one or more layers of copper traces on the surface or sandwiched between layers of substrate. To prevent corrosion and to prevent short circuits, the copper traces are coated with a thin film of “solder resist” except at the locations of pads or holes where components are to be soldered to the copper traces. In some cases, the copper traces may be gold plated.

In some cases, PCBs also contain resistive carbon traces printed on one or both sides of the PCB. The resistivity of such traces may vary between a few ohms/square and several kilo ohms/square. Such carbon traces may be used for a variety of purposes, including preventing corrosion of exposed copper contacts and to implement a variable resistance in combination with an external actuator or wiper.

The cost of a PCB is determined primarily by its area, the type of substrate material used, the number and size of holes in the PCB, and the number of layers of copper traces. The minimum width of the traces, and the minimum distance between traces also may significantly affect PCB cost, but the number of traces, or the percentage of the area of the PCB that is covered in copper are not significant factors affecting the cost of a PCB.

FIG. 2 shows a conventional actuator button 200 such as one used in a gamepad or other control device. The button has a carbon-impregnated domed rubber actuator, which makes contact with a resistive carbon PCB track. As the button is pressed harder, the rubber dome deforms, progressively shorting out more of the printed carbon PCB track, reducing the end-to-end resistance of the track, as shown in FIG. 2.

When the button is in the ‘rest’ position 210, it is not in contact with a carbon track 250, and resistive value of the track is shown as the resistor representation 260. When the button is gently pressed it goes to position 220, where the tip of the dome contacts the carbon track 250, and shorts across a small portion of the track 250. This is visible as the ‘shorted out’ portion of the resistor representation 265. When the button is pressed more firmly as shown in position 230, the tip of the dome deforms to become flatter and shorts out a larger portion of the track 250. This is visible as the wider ‘shorted out’ portion of the resistor representation 270. Finally, if the button is pressed hard as shown in position 240, the tip of the dome deforms to become quite flat and shorts out a much wider portion of the track 250, such that almost the entire track 250 is shorted out. This is visible as the widest ‘shorted out’ portion of the resistor representation 275.

The arrows in the drawing show the portion of the track which is not shorted out, and which is therefore resistive. The area between the arrows shows the area of the track which is shorted out. It can therefore be seen that as the rubber button is pressed harder, more of the track is shorted out, and the total resistance between the 2 ends of the track is reduced. The resistive track usually has a total resistance of a few kilo ohms, while the resistance of the conductive coating on the bottom of the rubber button is typically a few ohms at most. The resistance may be measured by placing a second resistor (for example 10K Ohms) in series with it to form a potentiometer, and measuring the output voltage from the potentiometer using an analog to digital converter (ADC).

This conventional actuator button and resistive track of FIG. 2 is somewhat less accurate than the force sensing resistor (FSR) approach, and has lower linearity. The main reason for the lower accuracy and non-linearity of the conventional actuator button and resistive track is the difficulty in printing a resistive track with a consistent resistivity along its length, and consistent resistivity from printed track to printed track. It is difficult to accurately control the thickness of the printed trace in a mass manufacturing process. However, absolute accuracy and linearity may not be important in many applications, and with calibration it is possible to give reasonably consistent results. Firmware may be used to calibrate for the non-linearity and also to calibrate for the changes in resistance as the rubber dome wears out with use. However, while the conventional actuator button and resistive track solution is less expensive than a force sensing resistor, it still costs several cents for each printed resistive trace on the PCB, and such costs can be significant in a consumer product with many force-sensing buttons (for example twelve buttons in the example of FIG. 1).

FIG. 3 shows a disassembled conventional gamepad 300, with resistive carbon traces 310, and conductive rubber dome actuators 320.

FIG. 4 shows a printed circuit board layout 400 of the conventional gamepad. The layout 400 shows resistive carbon printed traces 410, PCB traces 420, and solder resist (in this case blue, generally green in color) 430.

It would be desirable to have a less expensive force sensing button. A preferred force sensing button would be “free” (apart from the cost of the actuator itself) and provide linear sensing of force, with absolute accuracy that was consistent after calibration (low drift).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional gamepad device.

FIG. 2 illustrates a conventional actuator button.

FIG. 3 illustrates a disassembled conventional gamepad.

FIG. 4 illustrates a printed circuit board layout of the conventional gamepad layout.

FIG. 5 illustrates a side view of an improved force sensing actuator.

FIG. 6 illustrates a plan view of an improved force sensing actuator FIG. 7 illustrates an alternative embodiment of the improved force sensing actuator.

FIG. 8 illustrates another embodiment of the improved force sensing actuator.

FIG. 9 illustrates another alternative embodiment of the improved force sensing actuator

FIG. 10 illustrates another alternative embodiment of the improved force sensing actuator

DETAILED DESCRIPTION

Described is a solution for a force sensing actuation that uses the electrical properties of a printed circuit board, together with a conductive-tip actuator as to make a force-sensing button at extremely low cost.

FIG. 5 shows a side view 500 of the improved solution. The improved solution comprises a rubber actuator dome 510 which has a conductive layer (in one embodiment carbon) on the surface. In another embodiment, the entire actuator dome could be formed of conductive flexible material, or be impregnated with conductive material. The rubber actuator dome 510 is positioned above a PCB substrate 530. A conductive layer 550 is formed on the PCB substrate 530, and an insulating solder resist layer 520 is formed over the conductive layer 550. In one embodiment, the conductive layer 550 is a PCB trace comprising copper or an alloy thereof. A trace 540 is formed on a lower layer or on the opposite side of the PCB from the conductive layer 550 and the solder resist 520. The trace 540 is electrically isolated (i.e. not shorted to) from the conductive layer 550, the trace 540 forms a contact 545 on the PCB on the same side as the solder resist 520. The contact 545 is not fully covered by solder resist 520, such that any conducting material pressed down onto the top surface of the substrate will make electrical contact with contact 545. In another embodiment the contact 545 is exposed (i.e. there is no solder resist over it).

FIG. 6 shows a plan view 600 of the arrangement of FIG. 5. Plan view 600 shows the PCB trace 550 (in one embodiment in a circular shape, but could have any shape). Located between the edges of the PCB trace is the contact 545. In one embodiment this may be located approximately in the center of the PCB trace 550. Trace 540 is shown as a dotted line, this trace will be electrically connected to the rubber actuator dome 510 (which is not shown in the plan view) when the dome is pressed into contact with the substrate. Trace 560 is the trace from the lower electrode which is coupled to conductive PCB trace 550.

The actuator 510 is formed of, impregnated with or coated with a conductive material with a low resistivity, for example carbon. The rubber actuator dome may be the same type as used in conventional solutions. Solder resist is commonly used to coat the copper traces of a PCB to protect it from short circuits and oxidation and is of relatively uniform thickness and reasonably constant relative permitivity, with a value of approximately 4 in one example.

The value of the capacitance between two parallel plates is calculated as the permitivity of the material between the plates (the dielectric) multiplied by the overlapping area of the two plates, divided by the distance between the plates. Permitivity is commonly specified as two parts the permitivity of free space (epsilon-0 or E₀) and the relative permitivity of a particular material (gas, liquid, solid) known as epsilon-r or E_(r). Thus, the permitivity (epsilon) is E₀*E_(r).

A capacitor may be formed by the combination of a copper trace 550 (which acts as a lower plate), the solder resist 520 (which acts as a dielectric) and the conductive (e.g. carbon-printed) rubber actuator dome (which acts as an upper plate). As the actuator 510 is pressed down onto the PCB it will make contact with trace 540 through contact 545; as the actuator is pressed down with greater force, it will deform and a greater area of the conductive button will come into close proximity with the lower plate 550, thus increasing the capacitance between plate 550 and trace 540. A circuit on the board can be used to measure this capacitance. The output to be measured is a frequency that varies with capacitance. One example of such a circuit used to measure capacitance is a relaxation oscillator; this and other circuits for accurately measuring or detecting small changes in capacitance will be familiar to one skilled in the art. A processing element may read the output of this circuit and thus infer the force with which the button is being pressed.

The shape of the conductive trace 550 or the solder resist 520 can be varied while preserving the function of the invention. In order to maximize the capacitance between the actuator 510 and the trace 550, the trace 550 should generally cover the full area of contact of the actuator with the substrate when pressed with maximum force. In various configurations, the shape could be circle, square, rectangle, triangle, or any combination of these or other shapes. The shape could have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more sides, depending on how PCB layout software implements the conductive trace. PCB design/layout software may approximate a circular shape with a many sided shape, as true curves may be difficult to implement in PCB layout software. The conductive trace 550 may completely surround the contact 545, or may partially surround (such as a horseshoe shape) the contact 545. The conductive trace 550 may also be formed as a plurality of pieces (such as a pie chart shape) surrounding or partially surrounding the contact 545. The contact 545 may be located somewhere inside the limits of trace 520; generally the contact 545 should be located at or close to the point on the substrate where the actuator first touches the PCB, i.e. where the actuator touches when pressed with least force.

The improved solution operates in the following manner. In a first step when the actuator 510 is first touched by a user, it touches the sensor contact 545 which connects the actuator dome 510 to trace 540. In one example, trace 540 may be connected to electrical ground, such that dome 510 becomes grounded when it touches contact 545. This creates a small capacitance between the trace 550 and a ground voltage coupled to trace 540 and contact 545. In a second step when the actuator is pressed more firmly it deforms and approaches a wider surface of the trace 550 causing the capacitance between trace 550 and electrical ground to increase. In a third step, a circuit measures the capacitance. In a fourth step a microcontroller samples the circuit output and determines the capacitance value. In a fifth step, a digital representation of that capacitance value is generated. In one embodiment, this digital representation may be a six bit or eight bit value.

FIG. 7 shows an alternative embodiment 700 of the improved solution. In the embodiment 700, a first trace 710 is formed in close proximity to a second trace 720. Second trace 720 is coupled to ground. The traces 710 and 720 are electrically isolated, i.e. they are not shorted out. A layer of solder resist may be used to cover traces 710 and 720. The actuator 510 in combination with the first trace 710 and second trace 720 and solder resist 520 form a three plate capacitor, with two plates 710 and 720 side by side and the actuator acting as the third plate. In this embodiment the actuator does not make DC contact with either plates, allowing easier mechanical alignment during manufacturing, but may reduce the possible capacitance between the plates. Trace 710 is coupled to the measurement device.

FIG. 8 shows a further alternative embodiment 800 of the improved solution. In the embodiment 800, a first trace 810, a second trace 820 and a third trace 830 are formed. The first trace 810 is larger than either the second trace 820 or the third trace 830. The third trace 830 is coupled to ground. The second trace 820 is coupled to a logic input and the first trace 810 is coupled to the measurement device. A layer of solder resist is formed over plate 810, but plates 820 and 830 are not covered by solder resist.

The embodiment 800 operates in the following manner. When the actuator makes contact with the plates 820 and 830, the conductive actuator shorts them out and forms a DC connection to ground between the plates, which is detected by the logic input. Thus, the embodiment 800 forms both a combination switch and force sensing button.

In another alternative embodiment 900 shown in FIG. 9, plate 910 is fully covered with solder resist 520, and plate 920 is fully uncovered. When actuator 510 is pressed against the substrate, the actuator 510 is therefore grounded and, and a 2-plate capacitor is formed by 910 and 510 with solder resist acting as the dielectric.

Another alternative embodiment 1000 is shown in FIG. 10. The embodiment 1000 comprises a first plate 1010, a second plate 1020, and a grounded trace 1030 placed between the first plate and the second plate. First plate 1010 and second plate 1020 are covered in solder resist, but trace 1030 is exposed (i.e. no solder resist). This embodiment 1000 is well suited for implementation on a single side PCB board. In another embodiment, a further trace 1040 is present and located between the first plate 1010 and second plate 1020, where trace 1030 is grounded and trace 1040 is a logic output.

Embodiments of the present invention are well suited to performing various other steps or variations of the steps recited herein, and in a sequence other than that depicted and/or described herein. In one embodiment, such a process is carried out by processors and other electrical and electronic components, e.g., executing computer readable and computer executable instructions comprising code contained in a computer usable medium.

For purposes of clarity, many of the details of the improved force sensing actuator and the methods of designing and manufacturing the same that are widely known and are not relevant to the present invention have been omitted from the following description.

It should be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Therefore, it is emphasized and should be appreciated that two or more references to “an embodiment” or “one embodiment” or “an alternative embodiment” in various portions of this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined as suitable in one or more embodiments of the invention.

Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. 

1. An apparatus for sensing a force, comprising: an actuator having a conductive deformable surface; a substrate having a first conductive trace and a second conductive trace; a housing coupled to the actuator and to the substrate, holding the actuator in proximity to the substrate; and a circuit for measuring a capacitance value.
 2. The apparatus of claim 1, wherein a force applied to the actuator moves the actuator in a perpendicular manner towards the conductive traces of the substrate, causing the deformable surface to deform and increase a surface area in proximity to the conductive trace on the substrate, where the capacitance between the actuator and one or both of the traces increases as increasing force is applied to the actuator.
 3. The apparatus of claim 2, wherein the first conductive trace is located approximately below a center axis of the actuator, and the second conductive trace is located around the first capacitive trace.
 4. The apparatus of claim 3, where the first conductive trace is coupled to ground, and the second conductive trace is covered with solder resist.
 5. The apparatus of claim 2, wherein the first conductive trace and second conductive trace are both located approximately below a center axis of the actuator, and both the first and second conductive traces are covered by solder resist.
 6. The apparatus of claim 5, wherein the first conductive trace is coupled to ground.
 7. The apparatus of claim 2, wherein the first conductive trace and second conductive trace are both located approximately below a center axis of the actuator, and a third conductive trace is also located approximately below a center axis of the actuator.
 8. The apparatus of claim 7, wherein the first conductive trace is covered by solder resist, the second conductive trace is coupled to a logic signal, and the third conductive trace is coupled to ground.
 9. The apparatus of claim 1 wherein the actuating surface makes an electrical contact between a plurality of conductive traces, and forms a capacitor between those traces and other traces from which the actuating surface is electrically insulated.
 10. The apparatus of claim 9 in which an electrical circuit is used to detect said electrical contact between said un-insulated circuit board traces.
 11. The apparatus of claim 1 in which the circuit for measuring capacitance comprises a relaxation oscillator.
 12. The apparatus of claim 1 further comprising a processing element configured to calculate the force of actuation from the magnitude of the capacitance measured.
 13. The apparatus of claim 12 further comprising a communication means between the processing element and a computer system.
 14. A method for determining the force with which an actuator is pressed comprising: providing an actuator with a conductive actuating surface which deforms when pressed against a flat surface, where the area of the conductive surface in contact with the flat surface increases as the actuating force increases; providing a circuit board with conductive traces having an insulating material over the traces, such that a capacitance is created between said traces that varies as the actuating surface is pressed against the insulator; measuring a first capacitance value between the traces on the circuit board; and providing the capacitance measurement to a processing element.
 15. The method of claim 14, further comprising measuring a second capacitance value between the traces on the circuit board and computing a measure of the force of actuation from first and second capacitance values.
 16. The method of claim 14, further comprising computing a measure of the force of actuation from the first capacitance values.
 17. The method of claim 14 further comprising: detecting whether the actuator is in contact with the flat surface; and measuring the capacitance between the circuit board traces only when the actuator has been detected as being in contact with said surface.
 18. The method of claim 14 further comprising communicating a measure of the force computed to a computer system.
 19. The method of claim 18 further comprising: when the actuator is detected as not being in contact with the surface, communicating this information to the computer; and when the actuator is detected as being in contact with the surface communicating a measure of the force computed to the computer system.
 20. A circuit for sensing an actuating force, comprising a first conductive trace coupled to a reference voltage; a second conductive trace electrically insulated from the first conductive trace, located proximate to the first conductive trace; and a sensor having a sensing input coupled to the second conductive trace, and having an output.
 21. The circuit of claim 20, wherein the output of the sensor indicates a measure of actuating force.
 22. The circuit of claim 21, wherein the output of the sensor comprises one of a frequency, a voltage, a pulse width modulated signal, or a current.
 23. An entertainment system, comprising: an entertainment device configurable to provide multimedia output; a handheld input device capable of providing user input to the console, wherein the input device is configurable to sense an actuating force using a capacitance measurement; and a communication protocol operating between the input device and the entertainment console.
 24. The system of claim 23, wherein the entertainment device is a games console.
 25. The system of claim 24 wherein the communication protocol operating between the input device and the entertainment console is a wireless communication protocol.
 26. The system of claim 24 wherein the communication protocol operating between the input device and the entertainment console is a wired communication protocol.
 27. The system of claim 23 wherein the a handheld input device is a game controller. 